Cathode-grid assembly for electron guns



April 7, 1964 c. H. MATTSON 3,128,407

' cATHoDE-GRID ASSEMBLY FOR ELECTRON GUNS Filed NOV. 14. 1960 United States Patent 3,128,407 CATHODE-GRID ASSEMBLY FOR ELECTRON GUNS Clarence H. Mattson, Manheim, Pa., assigner to Radio Corporation of America, a corporation of Delaware Filed Nov. 14, 1960, Ser. No. 68,817 Claims. (Cl. 313-82) This invention relates to electron guns for cathode ray tubes and particularly to the cathode-grid subassembly thereof.

Electron guns used in cathode ray tubes usually comprise a plurality of coaxial tubular electrodes including a control grid cup and a cathode sleeve mounted coaxially within the control grid cup. Usually the cathode is disposed through the central aperture of a ceramic washer and mechanically xed therein by circumferential beads crimped in the cathode which bear against the opposite flat faces of the washer. The ceramic washer is then secured to the grid cup as, for example, by retainer rings welded to the cylindrical wall of the grid cup.

Such cathode-grid assemblies have proved to be particularly troublesome when used in cathode ray tubes for color television applications wherein a plurality of electron guns are employed. In plural gun tubes for color television use, a predetermined balance of operational characteristics between all of the guns must be maintained in order to provide a desired luminescent color balance. This is particularly true with regard to the mixing of the three primary colors red, green, and blue in a predetermined ratio to provide white light output. In order to maintain the desired balance, the cathode operating temperature characteristics and the grid control characteristics must be quite uniform from cathode-grid assembly to cathode-grid assembly, and must remain that way during repeated use of the tube.

In the above-described crimped assembly, and in other types of frictionally or mechanically xed assemblies, as opposed to assemblies which are brazed, welded or the like, the individual parts of the assembly, when subjected to thermal cycling in normal use, may change their relative positions. Such relative movement of the various parts of the assembly occurs because the parts after being expanded by heating in normal tube operation may not return to their original relative positions when operation ceases and the assembly is allowed to cool. As a result of the relative shifting of parts, thermal conductivity, between the parts may change resulting in a change of cathode temperature characteristics, and cathode-to-grid spacing may change resulting in a change of grid control characteristics. Since such changes may occur nonnniformly as between assemblies, the operational balance between electron'guns is nonuniform resulting in an unbalance of color output. Thus, receiver components adjusted at the factory to provide color balance between the three guns must be later readjusted.

Although it has been proposed that the cathode be brazed to its ceramic support member to prevent the above-described problems, the proposed techniques and designs of such brazed assemblies have not been satisfactory. When the cathode is brazed to the ceramic support in a conventional cathode-grid assemblyV construction, a relatively high melting temperature brazing material must be used in order that the resulting braze withstand normal operating temperatures of the cathode. Because of the high temperature required to make the braze, the cathode is often damaged during the brazing step. Moreover, a direct brazing of the cathode to its ceramic support results in high thermal conductivity between these two elements and a resultant excessively high heat loss from the cathode to the ceramic during lCC tube operation. This, in turn, results in an undesirably low cathode operating temperature.

It is therefore an object of my invention to provide a novel cathode-grid assembly which obtains improved uniformity of operating temperature characteristics from assembly to assembly.

It is also an object of my invention to provide a novel cathode-grid assembly in which uniformity of operating temperature characteristics is not excessively affected by vibration and temperature cycling of the assembly.

Another object of this invention is to provide a new and improved cathode-grid assembly which contributes to improved maintenance of color balance in plural gun cathode ray tubes designed for color television applications.

Briefly, according to my invention, a cathode sleeve having one end closed and coated with electron emissive material is mounted at its other end to one end of a cathode support sleeve which concentrically surrounds the cathode sleeve. from the cathode sleeve except at that end of the support sleeve to which the cathode sleeve is xed. The opposite end of the support sleeve is brazed to a ceramic washer. The cathode sleeve is mounted concentrically within a grid cup by fixing the ceramic washer thereto, preferably by a mounting sleeve brazed to the ceramic washer and welded to the grid cup.

In the drawings:

FIG. 1 is a perspective view of a plural electron gun apparatus incorporating the invention; and

FIG. 2 is a longitudinal section view of one of the cathode grid subassemblies of the apparatus of FIG. 1.

In FIG. l there is shown an electron gun assembly for providing a plurality of electron beams. The gun assembly is of a type used in color television cathode ray tubes and includes three similar electron guns 10, 11, and 12. Each of the electron guns comprises a plurality of coaxially arranged tubular electrodes mounted in spaced relationship on a plurality of glass support rods 14. Each gun includes a cathode-control grid aS- sembly 16, a screen grid 18, a focusing electrode 20, and an accelerating electrode 22 which is fixed to a dynamic convergence cage 24.

FIG. 2 illustrates the cathode-control grid assembly 16 of any one of the electron guns 1t), 11, or 12 of FIG. l. According to my invention, the assembly 16 includes a cylindrical control grid cup 3i) and a tubular cathode sleeve 32 mounted coaxially within the grid cup 30. The grid cup 30 is provided with an end closure wall 34 having a central electron beam aperture 36 therein. The cathode sleeve 32 is closed at one end with a transverse Wall 38, which is coated with electron emissive material 4). The cathode sleeve 32 is oriented within the grid cup 30 with the emissive coating 40 adjacent the aperture 36 of the grid cup.

The cathode-grid assembly 16 also includes a cathode mounting structure 42 which supports the cathode sleeve 32 coaxially within the grid cup 30. The mounting structure 42 comprises a ceramic washer 44, an outer mounting sleeve 46, and an inner cathode support sleeve 48.

The ceramic washer v44 has provided, yon its flat surface facing the gridcup end wall 34, inner and outer spaced, concentric, annular, metalized surfaces 50 and 52. The metalized surfaces `5() and -52 are lsepa-rated by an annular groove 54 Iwhich serves to shadow a portion of the washer surface from cathode sublimation and thus prevent a continuous conductive deposit on the washer and resultant electrical leakage between the cathode and control grid. An annular .rid-ge 55 on the opposite flat face of the washer l44 is also provided to= prevent electrical leakage.

The support sleeve is radially spaced The mounting sleeve i5 coaxially surrounds the ceramic washer 44 land is provided with an inwardly radially directed iiange 58 at one end thereof which is brazed to the outer metalized surface 52. The support sleeve 4? extends coaxially through the central aperture of the ceramic washer 44 and is provided :with an outwardly radially directed fia-nge @il which is brazed to the inner metalized surface Sit.

The cathode support sleeve 43 is formed with a first axial section 62 adjacent its fiange 69 which has a diameter substantially equal to the diameter of the central aperture to the cer-amic washer 44. Thus, the support sleeve 48 may be snugly fitted into the aperture of the ceramic rwasher 44 to automatically self-align the support sleeve '48 with the washer 44. The first `axial section 62 has a length substantially less .than the thickness of the washer The cathode support sleeve del is further formed wi-th a second axial section d4 adjacent to the rst section 62 which has a diameter somewhat less than the diameter of the first section 62. By virtue of the reduced diameter of the second section ed and the fact that the first section 62 is shorter than the thickness of the washer 44, contact |between .the cathode support sleeve t8 and the ceramic washer 44 is minimized. This, in turn, minimizes thermal conductivity between these two parts and thus contributes to efficient cathode operation.

The end of the cathode support sleeve 48 opposite its flanged end dit is provided with a still further reduced diame-ter section 66, the smallest diameter portion of which is substantially equal to the diameter of the cathode sleeve 32. The cathode sleeve 32 is mounted coaxially within its support sleeve d8 by welding its end opposite the emissive coating 4t? to the support sleeve 4S at this Small end 65. The cathode sleeve 32 and its support sleeve 48 are radially spaced from each other except at their weldedtogether ends.

The cathode sleeve 32 and the cathode support structure 42 are coaxially fixed within the `grid cup Sil by a final spot welding of the -mounting sleeve 46 to the cylindrical wall of the grid cup 30.

By virtue of my invention, wherein an entirely brazed and welded cathode-grid assembly is provided as opposed to prior art mechanically assembled structures, the problems encountered in such prior art structures are obviated or relieved. The brazed construction provides -a fixing together of the various parts which do not 'become loose due to thermal cycling and thus allow subsequent shifting of the parts. Temperature cycling, as encountered in normal operation of the tube into which the cathode-grid assembly =16 is incorporated, does not change thel static relative position `of the various parts ofthe assembly. When the assembly is heated .and thus expanded, and then subsequently allowed to cool and contract, the various parts of the -assembly return to their original positions rela-tive to each other. Likewise, vibrations which may be encountered do not cause the various parts of the assembly to change their relative positions. Thus, the cathode-grid assembly l# exhibits extremely Well-stabilized temperature and other operational characteristics. At thesame time a plurality of such assemblies possess extremely good uniformity of characteristics from assembly to assembly.

Unlike prior art attempts to provide brazed cathodegrid assemblies, the incorporation of, and the specific arrangement of, the cathode support sleeve 48 permits the necessary brazes to be -made Without the least bit of damage to the cathode 32. This feature results from the fact that, in the fabrication of the cathode-grid assembly 116, the mounting sleeve 46 and the cathode support sleeve 48 are first brazed to the ceramic washer 44. `After this subassembly is cooled `from its brazing Itemperature, the cathode sleeve 32 can be welded thereto. iFinally, the cathode sleeve 32, together with its support structure 42, can be telescoped into the grid cup 30 and then fixed thereto by a final welding of the mounting sleeve 46 to the grid cup 30. Alternatively, of course, after the cathode support structure 4t2 has been brazed together, i-t may be secured to the grid cup 3d by welding the mounting sleeve lr6 thereto; the cathode sleeve can then be telescoped within its support sleeve `48 and fixed by a final welding between the cathode sleeve 32 and its support sleeve 43.

Moreover, by virtue of the specific arrangement wherein the cathode support sleeve d8 contacts the cathode sleeve 32 at the end opposite the emissive coating 40, and is brazed to the ceramic ywasher at its end opposite the end to which the cathode sleeve 32j is mounted, adequate heat isolation is provided between the emissive coating 40 and the relatively large heat sink provided by the ceramic washer `414i and th'e grid cup 3ft.

Furthermore, inasmuch as the cathode sleeve 32 is not directly brazed -to the ceramic washer 4d, extremely high temperature braze materials need not be used. Accordingly, the necessary brazes may be provided economically and through the use of conventional techniques using, for example, copper brazes.

Another advantage, of the improved cathode-grid assembly lr6, is that it combines with the advantages named above the additional advantage of a unitary cathode-control grid subassembly. It is well Iknown in the industry that a unitary cathode-control grid subassembly offers the best-known rigidity of mounting of these electrodes. Other types of heat isolation mounting structures of the prior art, wherein the cathode is fixed to the lead-ins of the electron tube or onto insulator support rods onto which the grid cup is also mounted, tend to be microphonic. Thus, according to the invention an extremely rigid, yet stable and uniform, cathode-control grid assembly is provided.

I claim:

l. In an electron gun, a cathode-grid assembly comprising a tubular cathode sleeve closed at one end, a cathode support sleeve coaxially surrounding said cathode sleeve and fixed to said cathode sleeve at a point along said cathode sleeve longitudinally spaced from said one end, a ceramic washer having a metalized surface portion, said cathode support sleeve being brazed to said metalized surface portion at a point on said support sleeve longitudinally spaced from the point thereon at which said cathode sleeve is fixed, said cathode sleeve extending through the aperture of said washer, and a grid cup coaxially surrounding and fixed to said washer.

2. A cathodeagrid assembly for an electron gun, said assembly comprising a cylindrical grid cup having an aperture centrally in the end wall thereof, a cathode sleeve coaxially within said grid cup, the end of said cathode sleeve adjacent the aperture of said grid cup having a transverse lWall coated with electron emissive material, and a cathode support structure including a ceramic washer vvithin said grid cup coaxially surrounding said cathode, an outer sleeve coaxial with said washer and brazed at one of its ends tto said washer and fixed at its other end to the cylindrical wall of said grid cup, and an inner sleeve surrounding said cathode sleeve and brazed at one of its ends to said washer and fixed Iat its other end to said cathode sleeve at the end of said cathode sleeve opposite said coated end, said cathode sleeve and said inner sleeve being radially spaced from eachother except at their tixedtogether ends.

3. In an electron gun a cathode-grid assembly comprising a tubular cathode sleeve closed at one end, a cathode support sleeve coaxially surrounding said cathode sleeve and fixed at one end thereof to the other end of said cathode sleeve, a ceramic washer having a metalized surface portion, said cathode support sleeve being brazed at its other end to said metalized surface portion and a grid cup coaxially surrounding and xed to said washer, said cathode-support sleeve having first and second longitudinal sections extending through and being surrounded by said washer, said first section being adjacent said brazed end and fitting snugly within the aperture of said Washer, said second section having a smaller diameter than said first section and being radially spaced from the aperture wall of said Washer.

4. A cathode-grid assembly for an electron gun, said assembly comprising a cylindrical grid cup having an aperture centrally in the end wall thereof, a cathode sleeve concentrically Within said grid cup, the end of said cathode sleeve adjacent the aperture of said grid cup having a transverse Wall coated with electron emissive material, and a cathode support structure including a ceramic Washer Within said grid cup coaxially surrounding said cathode and having on its at surface facing said grid cup end wall inner and outer spaced concentric annular metalized areas, an outer sleeve coaxially surrounding said washer and having at one end an inward radial flange 1 other end welded to said cathode sleeve at the end thereof opposite said coated end, said cathode sleeve and said inner sleeve being radially spaced from each other except at their Welded together ends.

5. A cathode-grid assembly according to claim 4 and wherein said inner sleeve has adjacent its ange a first axial section which has a diameter substantially equal to the diameter of the aperture of said washer and a second axial section adjacent said first axial section which has a smaller diameter than the diameter of said iirst axial portion, said first axial section being shorter than the thickness of said washer.

References Cited in the le of this patent UNITED STATES PATENTS 2,833,952 Hemphill et al May 6, 1958 2,888,588 Dichter May 26, 1959 2,963,608 Benda et al Dec. 6, 1960 OTHER REFERENCES RCA Technical Notes, TN No. 321, November 1959. 

1. IN AN ELECTRON GUN, A CATHODE-GRID ASSEMBLY COMPRISING A TUBULAR CATHODE SLEEVE CLOSED AT ONE END, A CATHODE SUPPORT SLEEVE COAXIALLY SURROUNDING SAID CATHODE SLEEVE AND FIXED TO SAID CATHODE SLEEVE AT A POINT ALONG SAID CATHODE SLEEVE LONGITUDINALLY SPACED FROM SAID ONE END, A CERAMIC WASHER HAVING A METALIZED SURFACE PORTION, SAID CATHODE SUPPORT SLEEVE BEING BRAZED TO SAID METALIZED SURFACE PORTION AT A POINT ON SAID SUPPORT SLEEVE LONGITUDINALLY SPACED FROM THE POINT THEREON AT WHICH SAID CATHODE SLEEVE IS FIXED, SAID CATHODE SLEEVE EXTENDING THROUGH THE APERTURE OF SAID WASHER, AND A GRID CUP COAXIALLY SURROUNDING AND FIXED TO SAID WASHER. 